Patchwork [RFC,V6,29/33] qcow2: Do not overwrite existing entries with QCOW_OFLAG_COPIED.

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Submitter Benoît Canet
Date Feb. 6, 2013, 12:32 p.m.
Message ID <1360153926-9492-30-git-send-email-benoit@irqsave.net>
Download mbox | patch
Permalink /patch/218632/
State New
Headers show

Comments

Benoît Canet - Feb. 6, 2013, 12:32 p.m.
In the case of a race condition between two writes a l2 entry can be written
without QCOW_OFLAG_COPIED before the first write fill it.
This patch simply check if the l2 entry has the correct offset without
QCOW_OFLAG_COPIED and do nothing.

Signed-off-by: Benoit Canet <benoit@irqsave.net>
---
 block/qcow2-cluster.c |    5 +++++
 1 file changed, 5 insertions(+)
Stefan Hajnoczi - Feb. 8, 2013, 11:03 a.m.
On Wed, Feb 06, 2013 at 01:32:02PM +0100, Benoît Canet wrote:
> In the case of a race condition between two writes a l2 entry can be written
> without QCOW_OFLAG_COPIED before the first write fill it.
> This patch simply check if the l2 entry has the correct offset without
> QCOW_OFLAG_COPIED and do nothing.
> 
> Signed-off-by: Benoit Canet <benoit@irqsave.net>
> ---
>  block/qcow2-cluster.c |    5 +++++
>  1 file changed, 5 insertions(+)

Is this a general qcow2 bug (without deduplication)?

I thought there is L2Meta wait code in qcow2_co_writev() that prevents
overlapping writes from executing at the same time?

Patch

diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
index fedcf57..c016e85 100644
--- a/block/qcow2-cluster.c
+++ b/block/qcow2-cluster.c
@@ -763,6 +763,11 @@  int qcow2_alloc_cluster_link_l2(BlockDriverState *bs, QCowL2Meta *m)
     for (i = 0; i < m->nb_clusters; i++) {
         uint64_t flags = 0;
         uint64_t offset = cluster_offset + (i << s->cluster_bits);
+
+        if (be64_to_cpu(l2_table[l2_index + i]) == offset) {
+            continue;
+        }
+
         /* if two concurrent writes happen to the same unallocated cluster
 	 * each write allocates separate cluster and writes data concurrently.
 	 * The first one to complete updates l2 table with pointer to its